Question

Draw Newman projections for the staggered and eclipsed conformations of pentane for rotation about the \(\mathrm{C} 2-\mathrm{C} 3\) bond. Which conformation is lowest in energy?

Short answer

The staggered conformation of pentane is lowest in energy due to the absence of torsional strain, making it more stable than the eclipsed conformation.

Step-by-step solution

Understanding Pentane Structure

Pentane is a linear hydrocarbon with five carbon atoms (\(\mathrm{C}1-\mathrm{C}2-\mathrm{C}3-\mathrm{C}4-\mathrm{C}5\)) and enough hydrogen atoms so that each carbon has four bonds.

Drawing Staggered Conformation

Staggered conformation is one where the atoms or groups attached to two adjacent carbon atoms are as far apart as possible. Around \(\mathrm{C}2-\mathrm{C}3\) bond, draw a Newman projection. The three hydrogen atoms connected to \(\mathrm{C}2\) should alternate with the two hydrogen atoms and one \(\mathrm{C}4\) connected to \(\mathrm{C}3\). This represents the staggered conformation of pentane.

Drawing Eclipsed Conformation

Eclipsed conformation is one where the atoms or groups attached to two adjacent carbon atoms line up with each other. Similarly, draw a Newman projection around \(\mathrm{C}2-\mathrm{C}3\) bond. The three hydrogen atoms connected to \(\mathrm{C}2\) should align with the two hydrogens and one \(\mathrm{C}4\) to which \(\mathrm{C}3\) is bonded. This shows the eclipsed conformation of pentane.

Comparing Energy Levels

Eclipsed conformation is less stable and thus higher in energy due to the torsional strain. The hydrogens that line up with each other can cause close contact between electron clouds, leading them to repel each other. Staggered conformation is more stable and lower in energy as there is no torsional strain, and the electron clouds are as far apart as possible.

Key concepts

Staggered Conformation

In organic chemistry, conformations refer to the different spatial arrangements of atoms around a bond, specifically a carbon-carbon bond. Staggered conformation is a key arrangement where the atoms or groups bonded to two adjacent carbons are as far apart from one another as possible. This results in minimal steric repulsion between them. When viewing from the perspective of a Newman projection,:

• The front carbon typically displays groups like hydrogens spread out evenly apart.
• On the back carbon, the atom groups are seen in the gaps of those at the front.

In the case of pentane's rotation about the \( \mathrm{C} 2-\mathrm{C} 3 \) bond, when arranged in staggered conformation,:

• The hydrogen on the front carbon aligns with the spaces between the hydrogens and the carbon (connected to hydrogen and carbon in other positions) in the back carbon.
• This layout leads to reduced electron cloud interactions, making it energetically favorable.

The staggered formation is known to be lower in energy compared to its counterpart, the eclipsed conformation, due to these reduced interactions.

Eclipsed Conformation

The eclipsed conformation is another possible arrangement of atoms around a carbon-carbon bond but is not as energetically favorable as the staggered conformation. In eclipsed conformations:

• The atoms or groups attached to the two adjacent carbon atoms are aligned, or "eclipsing" each other, in a Newman projection.
• This creates a direct overlap of the atoms or groups, leading them to be as close together as possible.

Visualizing this with pentane's \( \mathrm{C} 2-\mathrm{C} 3 \) bond,:

• The hydrogens on the front carbon completely cover up the atoms attached to the back carbon.
• Such overlap maximizes the electron cloud interactions, leading to an increase in energy due to repulsion.

Thus, the eclipsed conformation of a molecule like pentane is typically less stable, explaining why such an arrangement is higher in energy than its staggered counterpart.

Torsional Strain

Torsional strain plays a crucial role in the relative stability and energy levels between different molecular conformations. It arises from the resistance experienced when rotating around a bond because of overlapping electron clouds in nearby atoms or groups. When molecules are in an eclipsed conformation, torsional strain increases because:

• The atom groups are aligned directly with each other, causing the clouds of electrons to repel.
• This repulsion leads to a less favorable energetic state.

In contrast, the staggered conformation mitigates torsional strain significantly due to:

• Atom groups being positioned further apart, reducing interactions of their electron clouds.
• The decreased electronic repulsion, which makes staggered states more stable and lower in energy.

Understanding torsional strain aids in predicting not only the conformation that a molecule might prefer energetically, but also its potential reactions and interactions with other molecules in a chemical environment.